The A Series is the most common paper size series that is defined in ISO216. The standard and most recognized paper size, used in the majority of offices around the world and by most home printers is the A4. This size has become the standard business letter size across the globe. But A4 is just one of the A-size papers. The A series paper sizes are in common use throughout the world apart from in the US, Canada and parts of Mexico.
The A series paper sizes are defined by the following requirements:
- The length divided by the width is 1.4142
- The A0 size has an area of 1 square metre.
- Each subsequent size A(n) is defined as A(n-1) cut in half parallel to its shorter sides. That means: A1 is half of A0, A2 is half of A1, A3 is half of A2, A4 is half of A3, A5 is half of A4 and so on.
- The standard length and width of each size is rounded to the nearest millimetre.
The table below shows you the size in mm and in of each of the A-size papers, A3,A4,A5, being the most common.
| Size | Width x Height (mm) | Width x Height (in) |
| 4A0 | 1682 x 2378 mm | 66.2 x 93.6 in |
| 2A0 | 1189 x 1682 mm | 46.8 x 66.2 in |
| A0 | 841 x 1189 mm | 33.1 x 46.8 in |
| A1 | 594 x 841 mm | 23.4 x 33.1 in |
| A2 | 420 x 594 mm | 16.5 x 23.4 in |
| A3 | 297 x 420 mm | 11.7 x 16.5 in |
| A4 | 210 x 297 mm | 8.3 x 11.7 in |
| A5 | 148 x 210 mm | 5.8 x 8.3 in |
| A6 | 105 x 148 mm | 4.1 x 5.8 in |
| A7 | 74 x 105 mm | 2.9 x 4.1 in |
| A8 | 52 x 74 mm | 2.0 x 2.9 in |
| A9 | 37 x 52 mm | 1.5 x 2.0 in |
| A10 | 26 x 37 mm | 1.0 x 1.5 in |
The A Series paper size chart, below gives a visual representation of how the sizes relate to each other – for example A5 is half of A4 size paper and A2 is half of A1 size paper.
RA & SRA Untrimmed Sizes. RA & SRA sizes define untrimmed paper for commercial printing. These formats are designed to allow for ink bleed during the printing process so that the paper can then be trimmed to one of the A series sizes.
Source: http://www.papersizes.org/